The long-term objective of this research is to gain an understanding of the cellular mechanisms by which neurons in the auditory cortex (ACx) process spectrally complex information. The hypothesis to be tested is that individual ACx neurons receive converging, largely subthreshold, inputs subserving most of the audible spectrum, and that synaptic integration produces optimal responses to complex stimuli. The proposed underlying neural architecture involves thalamocortical projections to ACx layer 3/4 mediating characteristic frequency (CF) and near-CF responses, and intracortical projections outside of layer 4 mediating spectrally-distant nonCF responses. The precise synaptic architecture dictates that optimal synaptic integration occurs in response to specific, predictable, spectrotemporally complex stimuli. Three Specific Aims utilize in vivo intracellular recordings and a newly developed in vitro auditory thalamocortical preparation: 1) Determine if subthreshold receptive fields are much broader than spike-based receptive fields, using in vivo intracellular recordings in ACx. 2) Determine if thalamic stimulation in vitro activates ACx layer 3/4 followed by excitation in upper and lower layers of adjacent ACx. In parallel in vivo experiments, determine if CF stimuli activate synapses in layer 3/4 whereas spectrally-distant nonCF stimuli activate synapses outside of layer 4. 3) Determine in vitro how converging synaptic potentials are optimally integrated, and then determine in vivo if similar integration underlies optimal responses to complex stimuli. An understanding of these mechanisms will enhance treatments for conditions such as partial ACx lesions or cochlear implants, by guiding the design of stimuli to activate ACx optimally. [unreadable] [unreadable]